This invention relates to spread spectrum communications, and more particularly to a matched filter which can be used for synchronizing to, and despreading of, a direct sequence, spread-spectrum signal.
Spread-spectrum communications require that an incoming spreading chip-code sequence embedded in a spread-spectrum signal, and the local spreading chip-code sequence at a receiver, be phase synchronized prior to processing of information transfer. Phase synchronization of the spreading chip code sequences is commonly known as code acquisition. Code acquisition is one of the most difficult issues facing the system designer.
Code acquisition is followed by the tracking process. Due to imperfect frequency references the incoming spreading chip-code sequence and the local spreading chip-code sequence tend to lose phase synchronization. Retaining the phase synchronization, or tracking, is a difficult process that typically employs feedback loops.
Conventional spread-spectrum systems implemented without the benefit of a matched filter employ additional circuits, such as delay locked loops (DLLs) dedicated to achieving and sustaining fine grained phase synchronization between the local spreading chip-code sequence and the incoming spreading chip-code sequence to within a unit of time which is less than the duration of one signal sample. The circuits for sustaining fine grain phase synchronization are difficult to design and implement.
In wireless environments, minimizing the performance degradation due to long or short duration attenuation of the incoming signal caused by changing propagation channel conditions is highly desirable. As the quality of the channel degrades, the quality of the detected signal degrades, often below acceptable levels.
Typical systems combat this condition by employing any of a variety of techniques collectively known as diversity processing. The diversity processing techniques have in common the ability to independently manipulate the information received through separate propagation paths, or channels, independently. The benefit from diversity processing is that when a given propagation channel degrades, the information can be recovered from signals received via other channels. A common though suboptimum, diversity technique is to employ two or more separate antennas and process the signal via two or more processing chains in parallel. Although necessary, the use of two or more antennas and processing is a difficult and costly undertaking, requiring two or more times the number of circuits required for one path as well as additional circuits and processing for insuring that the individual channel outputs are synchronized. A better approach is to employ a wideband signal of bandwidth W. If the multipath spread were TM then the receiver can recover L=TM(w+1) replicas of the incoming signal. If the receiver properly processes the replicas, then the receiver attains the performance of an equivalent Lth order diversity communication system. For wideband systems the value of L can become very large and it becomes unfeasible to implement L processing paths. Thus a non-matched filter spread spectrum receiver cannot attain the best possible performance.
The coherent demodulation of information signals requires that the phase of the carrier, at the radio frequency (RF), intermediate frequency (IF) or other frequency at which the demodulation is to take place, be known. The extraction of the phase of the carrier information requires that additional feedback loops be employed, such as phase-locked loops (PLLs), Costas loops, nth power loops or other devices capable of extracting the carrier phase information. In the wireless environment, where signals propagate through a multitude of separate and independent channels, each path processed by the receiver requires its own carrier phase information and therefore its own means to extract it. This requirement greatly increases the potential complexity of the system. The need to limit system complexity acts so as to limit the system performance.
Conventional receivers, for spread-spectrum reception or other coherent systems, employ circuits dedicated to extracting the carrier phase. These techniques, e.g., phase-locked loops (PLLS), Costas loops, n power loops, etc., exhibit design and implementation complexities that are well documented throughout the professional literature. A separate and independent set of these circuits is implemented for each individual signal path, or channel, that is received. Practical limits on system complexity force the system to receive a small subset of the L=TM(W+1) independent signal replicas.
A complex matched filter consists of two identical branches, in-phase (I) and quadrature (Q), used to process in-phase and quadrature signals. Each branch has a local signal reference register, an incoming signal register, a multiplication layer and an adder tree. The multiplication layer and the adder tree contained in the in-phase and quadrature branches are identical and may contain the majority of the gates used to implement the matched filter. To implement a matched filter it is preferable to reduce the size of the structure as much as possible.
Processing multiple signals, whether quadrature-phase-shift keying (QPSK) or binary-phase-shift keying (BPSK) modulated, simultaneously by matched filtering is desirable. An example of a requirement for processing multiple signals is the simultaneous matched filter processing of the I and Q components of the BPSK or QPSK spread-spectrum signal and then combining such signals. This normally requires the implementation of two or more matched filter structures, one per signal. Matched filters are large, costly and often difficult structures to build. Thus, limiting the size and complexity of the devices as much as possible is desirable.
A general object of the invention is a spread-spectrum receiver which reduces cost and circuit complexity, reduces the volume required and improves the performance, i.e., acquisition time, of conventional spread-spectrum chip-sequence signal acquisition.
Another object of the invention is a spread-spectrum receiver which has improved bit-error-rate (BER) performance over conventional coherent demodulation techniques, methods and circuits.
An additional object of the invention is to reduce cost and circuit complexity, reduce the volume required and improve the performance of conventional diversity reception, separation and combining techniques, methods and circuits.
A further object of the invention is to reduce the complexity associated with the simultaneous matched-filter processing of multiple signals, and yet not require a transmitted reference channel, such as a pilot channel.
According to the present invention, as embodied and broadly described herein, a spread-spectrum-matched-filter apparatus is provided including a code generator, a symbol-matched filter, a frame-matched filter, and a controller. The code generator is coupled to the symbol-matched filter and to the controller. The frame-matched filter is coupled to the output of the symbol-matched filter.
The spread-spectrum-matched-filter apparatus can be used as part of a spread-spectrum receiver, for receiving a received-spread-spectrum signal. A received-spread-spectrum signal, as used herein, is a spread-spectrum signal arriving at the input of the spread-spectrum receiver. The received-spread-spectrum signal is assumed to include a plurality of packets. Each packet has a header followed in time by data. The header and data are sent as a packet, and the timing for the data in the packet is keyed from the header. The data may contain information such as digitized voice, signalling, adaptive power control (APC), cyclic-redundancy-check (CRC) code, etc.
The header, or preamble, is generated from spread-spectrum processing a header-symbol-sequence signal with a chip-sequence signal. The data part of the packet is generated from spread-spectrum processing a data-symbol-sequence signal with the chip-sequence signal. The chip-sequence signal for spread-spectrum processing the header-symbol-sequence signal and the data-symbol-sequence signal are preferably, but do not have to be the same.
The code generator at a receiver generates a replica of the chip-sequence signal. The symbol-matched-filter has a symbol-impulse response which is matched to the chip-sequence signal of the received-spread-spectrum signal. The replica of the chip-sequence signal generated by the code generator is used to set or match the symbol-impulse response of the symbol-matched filter. when matched, upon receiving the received-spread-spectrum signal having the chip-sequence signal embedded therein, the symbol-matched filter can output the header-symbol-sequence signal and the data-symbol-sequence signal.
The replica of the header-symbol-sequence signal generated by the code generator can set the impulse response of the frame-matched filter to be matched to the header embedded in the received-spread-spectrum signal. Thus, the frame-matched filter has an impulse response, denoted herein as the frame-impulse response, which can be matched to the header-symbol-sequence signal. The start-data signal is triggered upon detecting the header in the frame-matched filter. Accordingly, the frame-matched filter filters the despread header and generates, as an output, a start-data signal in response to the header matching the frame-matched filter""s impulse response.
The controller controls to which of the symbol-impulse responses the symbol-matched filter is set. The controller can cause the symbol-impulse response of the symbol-matched filter to be matched to the chip-sequence signal of the received-spread-spectrum signal. Further, the controller can generate a plurality of symbol-control signals to cause the symbol-impulse response of the symbol-matched filter to be matched, sequentially, to a plurality of chip-sequence signals, respectively. Timing to the controller can be from the start-data signal generated at the output of the frame-matched filter. Thus, in response to the start-data signal received from the frame-matched filter, the controller can cause the symbol-matched filter to be matched to the chip-sequence signal using the replica of the chip-sequence signal. At a time delay from the start-data signal, triggered from the start-data signal, the controller can cause the output of the symbol-matched filter to be sampled for data symbols.
The present invention also includes a method for using a symbol-matched filter and a frame-matched filter as part of a spread-spectrum receiver on a received-spread-spectrum signal. As with the spread-spectrum-matched-filter apparatus set forth above, the received-spread-spectrum signal is assumed to include a header followed in time by data. The header and data are sent as a packet, and timing of the packet is triggered, for each packet, off the detected header.
The header is generated from spread-spectrum processing a header-bit-sequence signal with a chip-sequence signal. The data are spread-spectrum processed as a data-symbol-sequence signal with the chip-sequence signal. The chip-sequence signal for spread-spectrum processing the header-bit sequence signal and the data-symbol-sequence signal are preferably, but do not have to be, the same.
The steps include generating a replica of the chip-sequence signal, and programming, using the replica of the chip-sequence signal, the symbol-matched filter to have an impulse response matched to the chip-sequence signal. when the symbol-matched filter is matched to the chip-sequence signal, the method includes the steps of despreading the header from the received-spread-spectrum signal as a despread-header-symbol-sequence signal, and filtering with the frame-matched filter, the despread-header-symbol-sequence signal and generating a start-data signal.
When the symbol-matched filter is matched to the chip-sequence signal, the output of the symbol-matched filter is sampled at a time delay triggered from the detected header. The steps include despreading the spread-spectrum-processed data-symbol-sequence signal from the received-spread-spectrum signal as a despread-data-symbol-sequence signal.
Additional objects and advantages of the invention are set forth in part in the description which follows, and in part are obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention also may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.